A Level H1 Biology Genetics Inheritance Quiz

Answer Key - A-Level Biology H1 Quiz: Genetics Inheritance

Section A: Molecular Genetics

1. Rules: Adenine pairs with Thymine (A-T); Cytosine pairs with Guanine (C-G). [1] Significance: Hydrogen bonds are weak enough to allow the DNA strands to be separated (unzipped) for replication and transcription, yet strong enough collectively to maintain the double helix stability. [1]

2. Role: DNA polymerase catalyzes the formation of phosphodiester bonds between nucleotides [1]. It reads the template strand in the 3' to 5' direction and synthesizes the new complementary strand in the 5' to 3' direction [1]. It also performs proofreading to ensure accuracy [1].

3. Reason: DNA is too large to leave the nucleus and must be protected from degradation [1]. Transcription produces mRNA, which is a smaller, mobile copy of the gene [1]. This allows the original DNA to remain safe in the nucleus while multiple mRNA copies can be translated simultaneously in the cytoplasm to increase protein yield [1].

4. (a) 5'-AUG CCG UAA UCG-3' [1] (b) The DNA sequence changes from TAC to TGC. [1] This changes the mRNA codon from AUG (Methionine/Start) to ACG (Threonine). [1] This may prevent the translation from starting or change the primary structure of the protein, potentially altering its folding and function. [1]

5. Function: tRNA acts as an adapter molecule [1]. The anticodon recognizes and binds to the complementary codon on the mRNA [1], while the 3' end carries the specific amino acid corresponding to that codon, ensuring the correct sequence of amino acids is added to the polypeptide chain [1].

6. Missense: A point mutation that results in the substitution of one amino acid for another in the protein [1]. Nonsense: A point mutation that results in a premature stop codon, truncating the protein [1].

7. Degeneracy: Multiple codons can code for the same amino acid [1]. If a mutation changes a base but the new codon still codes for the same amino acid (silent mutation), the primary structure of the protein remains unchanged [1], and the protein's function is preserved [1].

Section B: Mendelian and Non-Mendelian Inheritance

8. Homozygous: Having two identical alleles for a particular gene (e.g., TT or tt) [1]. Heterozygous: Having two different alleles for a particular gene (e.g., Tt) [1].

9. Ratio: 1 Tall : 1 Short (or 50% Tall, 50% Short). [2]

10. Codominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood group in humans where both A and B antigens are present) [2]. Incomplete Dominance: The heterozygote shows an intermediate/blended phenotype (e.g., pink flowers from red and white parents) [2].

11. (a) Incomplete Dominance [1] (b) Symbols: R = Red, W = White. [1] Parents: RW x RW [1] Gametes: R, W and R, W [1] Offspring: RR (Red), RW (Pink), RW (Pink), WW (White). Ratio: 1 Red : 2 Pink : 1 White [1].

12. Man: rr (cannot roll) [1]. Woman: Rr (can roll, but must be heterozygous to produce a non-roller child) [2].

13. Explanation: According to the law of independent assortment, alleles for different traits segregate independently [1]. Each parent produces four types of gametes (AB, Ab, aB, ab) [1]. A 4x4 Punnett square results in 16 combinations [1], yielding the 9 (dominant-dominant) : 3 (dominant-recessive) : 3 (recessive-dominant) : 1 (recessive-recessive) ratio [1].

14. Epistasis: A phenomenon where one gene masks or interferes with the expression of another gene [2]. Difference: Simple dominance involves alleles of the same gene; epistasis involves the interaction between different genes [1].

Section C: Sex-Linkage and Complex Inheritance

15. Reason: Males are hemizygous for the X chromosome (XY) [1]. They only possess one copy of the X-linked gene [1]. Therefore, a single recessive allele on the X chromosome will be expressed, whereas females (XX) require two copies of the recessive allele to express the trait [1].

16. Probability: 50%. [3] (Female $X^RX^r$ x Male $X^RY$). Sons receive the Y from the father and either $X^R$ or $X^r$ from the mother. There is a 1/2 chance the son receives $X^r$.

17. Process: During Prophase I of meiosis, non-sister chromatids of homologous chromosomes pair up and exchange segments of DNA [2]. Importance: This creates new combinations of alleles on a single chromosome (recombinant chromosomes), increasing genetic diversity in gametes and subsequent offspring [2].

18. Mitosis: Produces two genetically identical daughter cells [1] that are diploid (2n) [1]. Meiosis: Produces four genetically unique daughter cells [1] that are haploid (n) [1].

19. Mode: X-linked Dominant [1]. Justification: Affected fathers pass the trait to all daughters because daughters must inherit the father's only X chromosome [1]. Affected fathers pass no trait to sons because sons inherit the Y chromosome from the father [1]. The presence in every generation suggests dominance [1].

20. Comparison: A substitution mutation only affects one codon [1]. A deletion mutation causes a frameshift [1]. This shifts the reading frame for all subsequent codons [1], leading to a completely different amino acid sequence [1] and often a premature stop codon, which almost always results in a non-functional protein [1].